This invention relates generally to CT and MR examinations of the heart, and more specifically to methods and apparatus for producing a long/short axis protocol from prescription images.
In at least one known computed tomography (CT) imaging system configuration, an x-ray source projects a fan-shaped beam which is collimated to lie within an X-Y plane of a Cartesian coordinate system and generally referred to as the “imaging plane”. The x-ray beam passes through the object being imaged, such as a patient. The beam, after being attenuated by the object, impinges upon an array of radiation detectors. The intensity of the attenuated beam radiation received at the detector array is dependent upon the attenuation of the x-ray beam by the object. Each detector element of the array produces a separate electrical signal that is a measurement of the beam attenuation at the detector location. The attenuation measurements from all the detectors are acquired separately to produce a transmission profile.
In known third generation CT systems, the x-ray source and the detector array are rotated with a gantry within the imaging plane and around the object to be imaged so that the angle at which the x-ray beam intersects the object constantly changes. A group of x-ray attenuation measurements, i.e., projection data, from the detector array at one gantry angle is referred to as a “view”. A “scan” of the object includes a set of views made at different gantry angles, or view angles, during one revolution of the x-ray source and detector. In an axial scan, the projection data is processed to construct an image that corresponds to a two dimensional slice taken through the object. One method for reconstructing an image from a set of projection data is referred to in the art as the filtered back projection technique. This process converts the attenuation measurements from a scan into integers called “CT numbers” or “Hounsfield units”, which are used to control the brightness of a corresponding pixel on a cathode ray tube display.
CT and MR (magnetic resonance) examinations now allow non-invasive imaging of the heart. They provide 3D data from which medical analysis or surgical planning may be performed. Image quality is quickly improving and 3D x-ray acquisitions may also soon provide the same information.
The most natural acquisition method (the only one in the case of CT) is along axial slices that do not correspond to a specific orientation of the heart, but just to horizontal planes. Displaying the heart along slices that contain its long or short axis is of great interest for assessing heart function. Such displays may be used to show the valves and the ventricles or to quantify the volume of the ventricles at different phases along the cardiac cycle. Acquiring MR slices in planes containing its long or short axis is a commonly used method, but requires an operator to have knowledge of cardiac anatomy and increases prescription time. In the case of CT, displaying slices in these planes has been done by post-processing of acquired CT data, because known CT scanners are capable only of acquiring data representative of axial slices.
It would therefore be desirable to provide methods and apparatus to facilitate the creation of a volume of slices along a short or long axis of the heart. It would also be desirable to make such methods readily accessible to untrained operators.